Thin-Wall YBCO Single Domains Oxygenated Under ... - IEEE Xplore

1 downloads 0 Views 422KB Size Report
Mar 8, 2013 - top-seeded melt growth process were successfully oxygenated by applying this annealing treatment. Under oxygen pressure of. 10 MPa, the ...
IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 23, NO. 3, JUNE 2013

7201005

Thin-Wall YBCO Single Domains Oxygenated Under Pressure: Optimization of Trapping Properties D. Kenfaui, X. Chaud, X. Hai, E. Louradour, and J. G. Noudem

Abstract—Thin-wall YBCO single-domain bulk materials present merits of reduced oxygen diffusion paths and large specific areas suitable for a fast oxygenation and a good thermal exchange. The progressive oxygenation under high pressure has been proposed to rapidly produce crack-free superconductors with stimulated trapping properties owing to speeding up of the oxygen diffusion. Thin-wall YBCO single domains grown by using top-seeded melt growth process were successfully oxygenated by applying this annealing treatment. Under oxygen pressure of 10 MPa, the influence of the dwell time t and temperature T on trapping properties was investigated. The trapped field was found to increase with t up to 12 h and reach an optimum at T = 700 ◦ C. A 700 mT trapped field has been obtained on a large thin-wall single domain (46 mm in diameter) classically oxygenated, while waiting the delivery of a suitable high oxygen pressure furnace for such size. Index Terms—Field trapping, mechanical properties, oxygen diffusion, progressive oxygenation, YBCO superconducting materials.

I. I NTRODUCTION

L

ARGE-GRAIN superconductors YBa2 Cu3 O7−δ (Y123) are potentially useful of the production of high trapped fields (> 2 T) in the 20–77 K temperature range by using a cryocooler or liquid nitrogen. They can be then used as permanent magnets [1], [2] or for compact and portable cryogenic motors and generators [2], [3]. The trapped field is a result of current loops flowing in the material, and thereby the trapping properties are directly governed by the critical current density, Jc , and the superconductor domain quality. Free grain-boundaries and textured Y123 samples, so-called single domains, can be reproducibly fabricated by using a Top-Seeded Melt Growth (TSMG) process [4]. Upon TSMG, the Y123 material is insulating and has a tetragonal structure with an oxygen content 7 − δ < 6.3 [5]. To reach the superconducting properties, the Y123 single domain is classically annealed at 400–500 ◦ C under 0.1 MPa oxygen flow in view of increasing oxygen content to 7 − δ > 6.8 [6], [7]. The crystalline cell Manuscript received October 8, 2012; accepted January 18, 2013. Date of publication January 25, 2013; date of current version March 8, 2013. This work was supported by the French National Research Agency (Agence Nationale de la Recherche-ANR) under Grant ANR-09-MAPR-002 (ASPAMEX Project) and Grant ANR-2010-BLAN-0944-02 (REIMS project). D. Kenfaui, X. Chaud, and X. Hai are with CNRS / CRETA-LNCMI, 38042 Grenoble Cedex 09, France (e-mail: [email protected]). E. Louradour is with CTI SA, 30340 Salindres, France (e-mail: eric. [email protected]). J. G. Noudem is with LUSAC/CRISMAT-CNRS UMR 6508, Université de Caen Basse-Normandie, 50130 Cherbourg, France (e-mail: jacques.noudem@ ensicaen.fr). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2013.2243196

parameters vary with the oxygen content leading to a tetragonal to orthorhombic transition around 500 ◦ C [7], which provokes a cracks network in cleavage (a, c) and (b, c) planes [8]. These cracks certainly contribute to promote material oxygenation, but they considerably hamper the current flow in the (a, b) planes, reduce the current loop size and thereby deteriorate the trapping properties. On the other hand, a continuous shortening of the c-parameter occurs when the Y123 material is cooled in air or oxygen from 900 to 100 ◦ C [8], which intensifies cracks into the material. Indeed, the oxygen diffusion rate is very low in the (a, b) plans (10−7 cm2 /s) [9], and, even worse, negligible along the c-axis because of a diffusion slower by 4–6 order of magnitude [10], which results in a c-parameter shrinkage over a very short distance. External thin oxygenated layers are consequently contracted and put in tension by the less oxygenated bulk material keeping the larger c-parameter. Such oxygen gradient induces a stress which causes cracks when that exceeds the material strength (18.4 MPa) [11]. Once the cracks are introduced, they easily spread throughout the whole sample, driven by the stress induced at the crack tip under the effect of the oxygen flow. These cracks mostly take place in the (a, b) planes. But branching along the c-axis, they also affect the current loops, and drastically impair the mechanical properties, limiting then the trapping capabilities. Y123 single-domain materials, with a brittle mechanical behavior, have been shown to fracture under the effect of the magnetic pressure when being subject to high-field activation [12]. Mechanical and thermal reinforcements are therefore decisive for trapping high fields. In fact, a trapped field larger than 14 T has been measured at 22.5 K on a Y123 superconducting pellet consolidated by a metal ring [12]. A value larger than 17 T have been recorded at 29 K between two 3 cm single-domain samples with a 1 mm hole introduced after growth and filled with metallic alloy [1]. To reduce the cracks, a modified annealing treatment, socalled progressive oxygenation, has been proposed [13]. This treatment aims to maintain the stress induced by the oxygen gradient below the Y123 material strength. Thin bars, cut from a Y123 single domain with a thickness below 1.5 mm parallel to the (a, b) planes, have been reported to be fully oxygenated by using this annealing treatment [13]. Thus, thinwall Y123 bulk materials have been developed [4] to make use of the progressive oxygenation. That consists in growing single domains on pellets being initially shaped with a regular network of holes (0.5–1 mm in diameter) spaced by 2.5 mm thickness walls. The diffusion paths are diminished and the material dimension that matters for oxygenation annealing is no longer the pellet diameter but the wall thickness. Moreover, this geometry provides larger specific areas for easier thermal

1051-8223/$31.00 © 2013 IEEE

7201005

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 23, NO. 3, JUNE 2013

Fig. 1. Oxygen rate of YBCO samples in the partial oxygen pressure versus temperature (diagram inspired from [6]). The blue arrows indicate the used progressive oxygenation.

exchange reducing the thermal gradients, and affords the possibility to impregnate the superconducting material through the holes for mechanical reliability reinforcement. However, a full progressive oxygenation with final dwell at 400–500 ◦ C requires very long time owing to the low oxygen diffusion at this temperature range, which limits the fabrication of Y123 bulk superconductors at industrial scale. It is then helpful to apply this treatment at higher temperature to increase the oxygen diffusion rate. That leads to a lower oxygen content, which involves the introduction of the oxygen high pressure in this process to speed up the oxygen diffusion by displacing the oxygen-temperature equilibrium towards the higher temperatures as illustrated in the diagram of Fig. 1. Here, we report on thin-wall Y123 single-domain materials fabricated by using TSMG process and progressively oxygenated under high temperature and pressure. The influence of the dwell time under these conditions and that of the dwell temperature on trapping properties are investigated and correlated to the microstructure features and the oxygen uptake. Larger thin-wall single domain of 46 mm in diameter was successfully grown and classically oxygenated, while waiting the delivery of a suitable high oxygen pressure furnace for such size. II. E XPERIMENTAL A. Large-Grain Fabrication Top-Seeded Melt Growth (TSMG) processing of singledomain YBa2 Cu3 O7−δ (Y123) bulk materials is detailed elsewhere [4]. Thin-wall pellets are prepared by compacting {70 %wt Y123 + 30 %wt Y2 BaCuO5 (Y211) + 0.15 %wt PtO2 in excess} powder under moderate uniaxial pressure (4.5 MPa) in 20 mm diameter die with embedded needles. The obtained preforms were put on an alumina-bearing plate with two intermediate buffer layers: a layer of Y2 O3 powder to absorb liquid loss and avoid reaction with alumina, and a 2 mm thickness disc of {80 wt% Y123 + 20 wt% YbBa2 Cu3 O7−δ } mixture to prevent undesirable nucleation from the preform bottom by introducing an element with lower melting point. The single domains are grown from a 2∗ 2 mm2 seed cut out Theva Gmbh films (a 200 nm NdBCO thin film deposited on MgO substrate). The seed acts as a nucleation center placed on top of the preform surface before TSMG processing in a furnace equipped with in situ video monitoring enabling a growth control. The samples are thereafter overheated in air at 1054 ◦ C for 2 h

to be peritectically decomposed into a liquid rich in barium and copper, and skeleton of solid Y211 inclusions. The Y123 recrystallization and the single-domain growth occur around 1004 ◦ C in a narrow solidification window where the samples are softly cooled (0.16 ◦ C/h) till the growth gets at the preform edge. The growth proceeds down to the samples bottom with a slight faster cooling (0.5–1 ◦ C/h). Once the single-domain samples have been grown, they are cooled under a nitrogen flow from 930 ◦ C to room temperature to prevent oxygen uptake leading to cell parameters change, and thereby to avoid cracks introduction into the material. B. Progressive Oxygenation Under Pressure It is expected to increase the oxygen content in Y123 single domain in a shorter time at a higher temperature under 10 MPa than under 0.1 MPa (Fig. 1) because of a higher diffusion rate. Similar oxygen content should be obtained at (520 ◦ C, 10 MPa) or (420 ◦ C, 0.1 MPa). However, full oxygenation at temperature higher than 600 ◦ C under 10 MPa is not predicted by this diagram. The high pressure oxygen annealing of thin-wall single domains is performed in a classical tubular furnace containing a vessel that can resist oxidization at high temperature and under pressure up to 16 MPa. The sample undergoes the progressive oxygenation under pressure by applying simultaneously temperature and pressure cycles and controlling the furnace atmosphere as follows: Firstly, the nitrogen gas is flown into the vessel to replace air at 30 ◦ C. The temperature is then continuously increased up to 900 ◦ C at a rate of 60 ◦ C/h under a flowing atmosphere (0.1 MPa) progressively changed from 100% nitrogen to 100% oxygen. The change is performed by using a program [13] that enables to follow the line x = 7 − δ = 6.3 (Fig. 1). The sample is maintained for 30 min at this temperature before being cooled to the dwell temperature T ; the oxygen pressure is introduced into the vessel at 850 ◦ C in two ramps: 0.2 MPa/h from 0.2 to 3 MPa, and 1 MPa/h from 3 to 10 MPa. The pressure is then maintained at 10 MPa for a dwell time t at the temperature T . To investigate the effect of the experimental parameters of the progressive oxygenation on trapping capabilities, a first series of ∼16 mm diameter thin-wall single domains is progressively oxygenated at 800 ◦ C under 10 MPa for dwell time t which is varied from 2 to 48 h. A second series was treated at different dwells temperature T , varied also from 640 to 800 ◦ C, under the same oxygen pressure for 12 h. III. R ESULTS AND D ISCUSSION A. Thin-Wall Single Domain and Microstructure Fig. 2 is a picture taken by a control camera showing a top view of the surface of three thin-wall and one plain pellets at an intermediate growth stage. One observes that for the drilled samples, the growth proceeds as for the plain one, and starts at a temperature of 1004 ◦ C. A square pattern in bright contrast indicates the growth front of the tetragonal Y123 structure following its a- and b-axis, starting from the seed to spread

KENFAUI et al.: THIN-WALL YBCO SINGLE DOMAINS OXYGENATED UNDER PRESSURE

Fig. 2. Surface of three drilled and one plain pellets at an intermediate stage during the growth processing as taken by in situ control camera.

Fig. 3. (a) Single-domain sample grown from a drilled pellet. (b) Y211 inclusions in the Y123 matrix.

on the whole surface of the pellets. The square pattern size is different for samples, evidencing a difference in the growth rate. That is ascribed to the holes network when comparing the plain and drilled samples, and probably to the temperature gradients in the furnace when comparing the drilled pellets to each other. We can notice also that the holes network does not disturb the growth, which extends over the entire surface of the pellets [Fig. 3(a)]. Fig. 3(a) illustrates top and side views of a thin-wall single domain. As for the plain one, the growth proceeded to the edge of the pellet along all its depth and the total process lasted about one week. The holes network is not distorted or destroyed and the holes remain open after TSMG processing; their diameter is reduced from 0.5 to 0.41 ± 0.01 mm. The pellets also shrink from 20 mm to 15.91 ± 0.19 mm in diameter. Fig. 3(b) shows the optical micrograph of mirror-polished surface parallel to the vertical c-axis of the single domain progressively oxygenated at 700 ◦ C under 10 MPa for 12 h. It evidences the typical microstructure features known from the melt-textured processed bulk Y123 materials. The Y211 inclusions in dark contrast seem to be uniformly distributed in the Y123 matrix (bright contrast). Two categories of the particles size are observed: small (majority) inclusions with size varying between 1 and 4 μm, and large (minority) ones that can get at 25 μm in length. However, the Y211 inclusions size appears larger than that reported by Chaud et al. [4], probably due to the initial Y211 powder since our samples were elaborated by using the same process. We note that all the samples were prepared from the same powder batch. Furthermore, we can clearly see that there is no porosity and no cracking network at contrast of the reported plain sample [4],

7201005

Fig. 4. 3-D representation of the surface induction measured on the sample oxygenated at 700 ◦ C under 10 MPa for 12 h.

Fig. 5. 2-D representation of the surface induction measured on the samples oxygenated (a) progressively at 700 ◦ C under 10 MPa for 12 h and (b) classically at 420 ◦ C under 0.1 MPa for 72 h.

which points out a considerable diminishing in stress related to the oxygen and thermal gradients thanks to the thin-wall geometry associated to the progressive oxygenation. B. Trapping Properties When Y123 bulk materials are cooled below the temperature of 92 K under a magnetic activation, they trap and keep a part of the applied field, and thereby act as permanent magnets as long as they are maintained at such temperatures. After oxygenation treatment, the samples were magnetized in field of 1.5 T provided by a superconducting coil. The remnant induction at the samples surface was then measured with a Hall probe. The scan is made at 77 K at a distance of 0.2 mm from the surface with a grid step of 0.5 mm. Flux mapping results obtained for a thin-wall sample oxygenated at 700 ◦ C under 10 MPa for 12 h are shown in Fig. 4. This 3D representation shows a typical shape of the trappedfield distribution in Y 123 bulk superconductors with a single regular peak, which points out the single-domain quality of the material. That is coherent with the macroscopic observations (Fig. 3). The current flows in a single loop at the scale of the thin-wall sample progressively oxygenated under pressure as shown from its trapped field 2 D representation [Fig. 5(a)]. That is a consequence of a drastic diminishing of cracks and pores, and an absence of residual grains. However, the 2 D representation of a thin-wall sample classically oxygenated [Fig. 5(b)] revealed a distorted current loop mainly caused by the cracks network introduced during the oxygenation annealing, which limits the current flow. Thus, the sample progressively oxygenated at 700 ◦ C under 10 MPa for 12 h trapped a magnetic field reaching a maximum value of 475 mT, for a diameter and

7201005

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 23, NO. 3, JUNE 2013

Fig. 6. Trapped-field versus (a) dwell time t and (b) dwell temperature T .

height of 16 and 10 mm, respectively. This performance is about 4 times larger than that measured on a plain sample (120 mT) with similar dimensions, prepared from the same powder batch, and classically oxygenated. Nevertheless, this value remains lower than that reported by Chaud et al. [4], perhaps due to the larger Y211 particles observed in our materials which probably lead to less efficient pining sites. The diagram in Fig. 6(a) gives the maximum trapped fields recorded on thin-wall samples (first series) progressively oxygenated at 800 ◦ C under 10 MPa for different dwells time t. All the samples revealed single-domain quality. The trapped field is monotonically improved as the dwell time t is increased up to 12 h, which can be likely due to an increase in oxygen uptake and twinning density as the twins act as pinning sites. The number of twin defaults was reported to be enhanced at high oxygen pressure [5]. For this series, the largest value of 450 mT is reached for dwell time of 12 h. Beyond 12 h, the trapped-field slightly decreases when the dwell time is increased to 24 h. This is probably due to oxygen saturation under longer annealing treatment, which may lead to a decrease of the default inhomogeneity and, consequently, the pinning sites. This explanation seems to be consolidated as the dwell time is doubled (48 h). Indeed, a loss of more than 50% in trapped field occurred. Fig. 6(b) shows no change in trapped field when the dwell temperature T decreases from 800 to 750 ◦ C under 10 MPa for the optimized dwell time (12 h). The trapped field is improved from 450 to 475 mT at 700 ◦ C, before being considerably reduced for the lower T of 640 ◦ C owing to lower oxygen uptake or cracks. We note that for each t or T values, three samples were measured to check the result reproducibility. Further experiments are still needed to achieve a good compromise between fast oxygen diffusion and default inhomogeneity. C. Large Thin-Wall Single Domain The trapping properties are proportional to the superconductor size. Bigger single domain can then trap larger magnetic field, which is more suitable for the industrial applications [1]–[3]. Larger thin-wall single domains of 46 mm in diameter and 15 mm in height (Fig. 7(a)) were successfully grown from 50 mm diameter drilled pellet by using TSMG process. The initial preforms were prepared by pressing the YBCO powder into a pellet, and holes of 1 mm in diameter are then automatically introduced by using a drilling machine. The single domains shrink from 50 to 46 mm in diameter. For quality test, some samples were annealed under an oxygen

Fig. 7. (a) View of large thin-wall single domain (46 mm in diameter) grown by using TSMG process. (b) Profile of the trapped field measured.

flow (0.1 MPa) at 400–500 ◦ C, while waiting for the delivery of a larger high oxygen pressure furnace suitable for this size. A sample cooled at 77 K under 2 T was characterized by the flux mapping with a larger grid of 36 mm by 36 mm. Its trapped field reached 700 mT, which is larger than the values reported on large single domains treated by similar oxygenation annealing [14]. The profile of the magnetic flux trapping [Fig. 7(b)] shows a single peak with deformed shape, due to cracks occurring during the classical oxygenation and to growth defects at the pellet edge. By applying the progressive oxygenation procedure on such pellets, we expect trapped field of at least 1.4 T if the improvement ratio observed on smaller samples could be applied. IV. C ONCLUSION We report the optimization of some parameters of the progressive oxygenation leading to high oxygen diffusion, suitable for a faster fabrication of thin-wall YBCO bulk materials fully oxygenated, and to sufficient default inhomogeneity useful for stimulating the trapping properties. At this stage, the largest trapped field of 475 mT was found at 700 ◦ C under 10 MPa for 12 h on 16 mm diameter sample, representing an improvement of about 4 times compared with the standard material. The optimization of other parameters (pressure, heating rate, etc.) is under way. A large thin-wall single domain of 46 mm in diameter was grown and classically oxygenated. An interesting trapped field value of 700 mT was obtained. The optimized thin-wall single domains will be reinforced with resin and metal/alloy for characterizations at lower temperatures (20–50 K) by using a cryocooler. R EFERENCES [1] M. Tomita and M. Murakami, “High-temperature superconductor bulk magnets that can trap magnetic fields of over 17 tesla at 29 K,” Nature, vol. 421, no. 6922, pp. 517–520, Jan. 2003. [2] Y. Ren, J. Liu, R. Weinstein, I. G. Chen, D. Parks, J. Xu, V. Obot, and C. Foster, “Quasi permanent superconducting magnet of very high field,” J. Appl. Phys., vol. 74, no. 1, pp. 718–719, Jul. 1993. [3] M. Morita, K. Magashima, S. Takebayashi, M. Murakami, and M. Sawamura, “Trapped field of YBa2Cu3O7 QMG bulk magnets,” Mater. Sci. Eng., vol. 53, no. 1/2, pp. 159–163, May 1998. [4] X. Chaud, J. Noudem, T. Prikhna, Y. Savchuk, E. Haanappel, P. Diko, and C. P. Zhang, “Flux mapping at 77 K and local measurement at lower temperature of thin-wall YBaCuO single-domain samples oxygenated under high pressure,” Phys. C, vol. 469, no. 15–20, pp. 1200–1206, Oct. 2009. [5] T. Prikhna, X. Chaud, W. Gawalek, J. Rabier, Y. Savchuk, A. Joulain, A. Vlasenko, V. Moshchil, N. Sergienko, S. Dub, V. Melnikov,

KENFAUI et al.: THIN-WALL YBCO SINGLE DOMAINS OXYGENATED UNDER PRESSURE

[6] [7]

[8] [9]

D. Litzkendorf, T. Habisreuther, and V. Sverdun, “Oxygenation of the traditional and thin-walled MT-YBCO in flowing oxygen and under high evaluated oxygen pressure,” Phys. C, Supercond., vol. 460–462, no. 1, pp. 392–394, Sep. 2007. H. M. O’Bryan, P. K. Gallagher, R. A. Laudise, A. J. Caporaso, and R. C. Sherwood, “Oxidation of Ba2 YCu3 Ox at High PO2 ,” J. Amer. Ceram. Soc., vol. 72, no. 7, pp. 1298–1300, Jul. 1989. E. D. Specht, C. J. Sparks, A. G. Dhere, J. Brynestad, O. B. Cavin, and D. M. Kroeger, “Effect of oxygen pressure on the orthorhombic-tetragonal transition in the high-temperature superconductor YBa2 Cu3 Ox ,” Phys. Rev. B, Condens. Matter, vol. 37, no. 13, pp. 7426–7434, May 1988. P. Diko, “Cracking in melt-grown RE-Ba-Cu-O single-grain bulk superconductors,” Supercond. Sci. Technol., vol. 17, no. 11, pp. R45–R58, Nov. 2004. M. Klaser, J. Kaiser, F. Stock, G. Muller-Vogt, and A. Erb, “Comparative study of oxygen diffusion in rare earth REBa2 Cu3 O7−δ single crystals (RE = Y, Er, Dy) with different impurity levels,” Phys. C, Supercond., vol. 306, no. 3/4, pp. 188–198, Sep. 1998.

7201005

[10] S. J. Rothman, J. L. Routbort, U. Welp, and J. E. Baker, “Anisotropy of oxygen tracer diffusion in single-crystal YBa2 Cu3 O7−δ ,” Phys. Rev. B, Condens. Matter, vol. 44, no. 5, pp. 2326–2333, Aug. 1991. [11] M. Tomita, M. Murakami, and K. Katagiri, “Reliability of mechanical properties for bulk superconductors with resin impregnation,” Phys. C, Supercond., vol. 378–381, no. 1, pp. 783–787, Oct. 2002. [12] G. Fuchs, P. Schatzle, G. Krabbes, S. GruB, P Verges, K.-H. Muller, J. Fink, and L. Schultz, “Trapped magnetic fields larger than 14 T in bulk YBa2 Cu3 O7−x ,” Appl. Phys. Lett., vol. 76, no. 15, pp. 2107–2109, Apr. 2000. [13] D. Isfort, X. Chaud, R. Tournier, and G. Kapelski, “Cracking and oxygenation of YBaCuO bulk superconductors: Application to c-axis elements for current limitation,” Phys. C, Supercond., vol. 390, no. 4, pp. 341–355, Jul. 2003. [14] X. Chaud, D. Bourgault, D. Chateigner, P. Diko, L. Porcar, A. Villaume, A. Sulpice, and R. Tournier, “Fabrication and characterization of thin-wall YBCO single-domain samples,” Supercond. Sci. Technol., vol. 19, no. 7, pp. S590–S600, Jul. 2006.